The challenges and limitations of vivianite quantification with 2,2'-bipyridine extraction

Vivianite presents a significant phosphorus pool in iron-rich, reducing environments, necessitating the development of an affordable and routine quantification method. A novel extraction protocol using 2,2'-bipyridine (Bipy) was proposed as a promising approach. However, the efficacy of this protocol in achieving complete vivianite extraction remains uncertain and lacks robust analytical validation. This study systematically assessed the Bipy extraction protocol on known amount of synthetic and two magnetically recovered environmental vivianite samples, with varying oxidation levels, impurity content, and particle size. Extraction efficiencies of iron and phosphorus were 13-44 % and 13-55 %, respectively, indicating incomplete extraction under the original protocol conditions (0.2 % Bipy, 24 h). An initially rapid release of iron and phosphorus slowed down across all samples, indicating non-constant reaction rates, and suggesting that the extraction is governed by mechanisms beyond kinetic control. Extending the extraction time to 48 h and increasing the Bipy concentration to 1 % yielded marginal improvements, with efficiency gains of 14 % or less. Grinding, which reduced particle size, nearly doubled extraction efficiency. Conversely, the sample with the highest Fe²⁺ content showed the overall lowest overall extraction efficiencies. Similarly, recovered vivianite samples containing impurities, namely magnesium and calcium, were extracted less efficiently than synthetic vivianite. Additionally, the extracted iron-to‑phosphorus ratio exceeded the theoretical value of 0.67, indicating non-stoichiometric extraction. To establish Bipy extraction as a reliable analytical method, it is crucial to address incomplete extraction and determine whether vivianite can be fully extracted or only to a certain extent. A potential strategy involves reducing extraction time and repeating the extraction step.